The invention relates to the selection of a cell carried out by the mobile station of a cellular radio system in general. In particular, the invention relates to the selection of a cell in a situation where there are cells with different qualities of service available.
A mobile station of a cellular radio system always tends to select a certain base station in the cell of which it operates or camps. Traditionally, the selection of a cell was based on the measurement of the strength of the received radio signal, either in the mobile station or the base station. In the GSM system (Global System for Mobile Telecommunications), for instance, each base station transmits a signal in a certain so-called BCCH channel (Broadcast Control Channel), the frequency of which is different at neighbouring base stations, and the mobile stations measure the strengths of the BCCH signals received and determine on the basis of the strength which cell is the most advantageous with regard to the quality of the radio connection. The base stations also transmit information about the BCCH frequencies used in neighbouring cells to the mobile stations so that they know which frequencies they must listen to in order to find the BCCH transmissions of neighbouring cells. In each cell, the transmission of the BCCH channel also contains information of how the mobile stations can make so-called random access requests in the cell for establishing a telephone connection.
The prior art arrangement is very suitable for the digital cellular radio systems of the second generation, such as GSM and its extension DCS1800 (Digital Communications System at 1800 MHz, IS-54 (Interim Standard 54) and PDC (Personal Digital Cellular). However, it has been suggested that in the digital cellular systems of the third generation in the future, the quality of service offered by the cells to the mobile stations varies substantially from cell to cell. Suggestions for systems of the third generation are UMTS (Universal Mobile Telecommunications System) and FPLMTS/IMT-2000 (Future Public Land Mobile Telecommunications System/International Mobile Telecommunications at 2000 MHz). In these plans, the cells are divided according to size and properties into pico-, nano-, micro- and macrocells, for example, and the data transfer speed can be used as an example of the quality of service. The picocells have the highest data transfer speed, and the macrocells have the lowest. The cells can be partly or entirely overlapping, and there can be different kinds of mobile stations, whereby all mobile stations may not be able to use the quality of service offered by all the cells.
FIG. 1 shows a likely future form of a cellular radio system, which is not entirely new as compared to the GSM, but which contains both known parts and entirely new parts. The bottleneck of the present cellular radio systems, which impedes offering more developed services to mobile stations, is the radio access network formed by the base stations. The core network of the cellular radio system, which consists of Mobile services Switching Centres (MSC), other network elements (in GSM, for instance, the SGSN and GGSN related to the packet switched radio systems, which mean Serving GPRS Support Node and Gateway GPRS Support Node, where GPRS means General Packet Radio Service) and transmission systems related to them, is capable, at least as conforming to the GSM Phase 2+ specifications, which are a further development of GSM, of transmitting the new kinds of services. In FIG. 1, the core network of the cellular radio system 10 is the GSM Phase 2+ core network 11, with three parallel radio access networks connected to it. Of these, the networks 12 and 13 are UMTS radio access networks and the network 14 is a GSM Phase 2+ radio access network. Of the UMTS radio access networks, the upper 12 in the figure is a general radio access network, which is owned by a teleoperator offering mobile station services and which serves equally all the customers who have a subscription with the teleoperator. The lower UMTS radio access network 13 is private, and its owner is, for instance, a large company in the premises of which the radio access network operates. The cells of the private radio access network 13 are typically nano- and/or picocells, and only the mobile stations of the company""s employees can operate in them. The cells of all the three radio access networks 12, 13 and 14 can be entirely or partly overlapping.
The mobile station 15 shown in FIG. 1 is preferably a so-called dual-mode mobile station, which can operate either as a GSM mobile station of the second generation or as a UMTS mobile station of the third generation, depending on what kind of services there are available in its location at the time and what the data transfer needs of the user are. It can also be a multi-mode mobile station, which can operate in many different data transfer systems according to the need and availability of services. A SIM unit 16 (Subscriber Identity Module) connected to the mobile station is also shown separately in the figure.
The application of the prior art arrangement to a digital cellular system of the planned third generation would mean that each base station would broadcast a transmission like the BCCH channel, which would include, in addition to other information, information of the BCCH frequencies of neighbouring and/or overlapping other cells. Because in the known systems each base station also expresses information of itself in its own BCCH signal for implementing the connection requests, an apparent solution would be adding to the own BCCH transmission of the third generation base station information of what quality of service the base station offers and to which class of size its cell belongs. However, an arrangement like this loads the mobile stations, because a mobile station operating in a certain cell must at first read from the BCCH signal transmitted by the base station of its cell a list of other BCCH frequencies, and thereafter receive, demodulate and decode a short period of each BCCH transmission coming at a frequency included in the list in order to find out if the mobile station can operate in the neighbouring cells in question. It can happen that a considerable part of the neighbouring cells are 2 Mbit/s picocells, but the mobile station can only operate at a data transfer speed of 64 kbit/s in macrocells, in which case it would not be worthwhile to waste time and electric power for receiving the BCCH transmissions of the picocells (it is even possible that the slow 64 kbit/s mobile stations cannot operate at all in the fast picocells; alternatively a slow mobile station can operate in a fast picocell only if there are no other cells available). Similarly, it can happen that a fast UMTS mobile station operating temporarily in a 64 kbit/s macrocell would want to establish a fast 2 Mbit/s data transfer connection, whereby it must change to one of the picocells available. If the quality of service of most of the neighbouring cells is too low for this, the reception, demodulation and decoding of their BCCH transmissions in order to find a new cell is wasted.
It is an objective of this invention is to present a method and a system in which the use of data transfer resources for finding a suitable new cell is more efficient than in the prior art solutions.
The method according to the invention, in which method the base station creates a message containing information about the neighbouring cells and transmits it to the mobile station, is characterized in that in addition to the transmission frequencies used in the neighbouring cells, the message of the base station also contains other information characterizing the neighbouring cells. In the second embodiment of the method according to the invention, it is characterized in that it comprises steps in which
a) the base station transmits to the mobile station a message which contains information describing the neighbouring cells near the base station,
b) on the basis of the message, the mobile station makes a preliminary selection of a group of the most advantageous new cells, and
c) the mobile station performs measurements among the preliminary selection of cells in order to find a suitable new cell, whereby
if the measurements show that the preliminary selection of cells contains at least one suitable new cell, the mobile station selects the new cell from among the preliminary selection of cells, and
if the measurements show that the preliminary selection of cells does not contain any suitable new cell, the mobile station selects the new cell from among other cells than those included in the preliminary selection.
The invention also relates to the base station and mobile station of the cellular radio system. The base station according to the invention is characterized in that it comprises information of a group of neighbouring cells near the cell of the base station in question, containing information of the transmission frequency used in the neighbouring cells and of the quality of service offered by the neighbouring cells to mobile stations, and it is equipped to create of this information a message describing the neighbouring cells and to transmit the message to mobile stations. A mobile station according to the invention is characterized in that it is equipped to express information about the quality of service of the cells from the messages transmitted by the base station of the cell selected at the moment, and to focus the measurements primarily on the neighbouring cells of the cell selected at the moment, which according to the information expressed correspond to the quality of service required by the mobile station.
The invention also relates to a cellular radio system, which comprises base stations cells related to them and mobile stations, and in which the base stations are equipped to create and transmit messages to the mobile stations, and the mobile stations are equipped to expect a certain maximum quality of service from the cell, to receive messages from the base stations and to perform measurements of the cells in order to find a suitable cell for a new selection. It is characterized in that it comprises in at least one base station information about a group of neighbouring cells near the cell of the base station in question, including information about the transmission frequency used in the neighbouring cells and about the quality of service offered to mobile stations by the neighbouring cells, whereby the base station is equipped to create of this information a message describing the neighbouring cells and to transmit this message to the mobile stations.
The operation of the mobile stations for finding a suitable new cell can be directed more efficiently than in the prior art solutions, if the mobile station knows the quality of service offered by other base stations in advance In the arrangement according to the invention, the base station transmits information about the quality of service of the base stations of the neighbouring cells, whereby in order to find a new cell, the mobile station receives, demodulates and decodes the transmissions of only those neighbouring cells in which the transmitted signal has a sufficient power level, and which on the basis of the information transmitted by the present base station have a suitable quality of service. According to need, the base station maintains lists of those surrounding base stations to a cell of which it can change in the near future. The number and mutual order of base stations in the lists maintained by the mobile station is determined by the capability of the mobile station to utilize the services offered by the surrounding base stations.